1. Field of the Invention
The invention pertains to a vibration damper with amplitude-selective damping force which includes a cylinder containing a damping medium, a piston rod which is axially movable in the cylinder, and a piston assembly arranged for axial movement in the damping medium, the piston assembly being connected to the piston rod and dividing the cylinder into a working space around the piston rod and a working space opposite the piston rod. The piston assembly includes a piston body having connecting channels for opposite directions of flow between the working spaces, and valve disks which open the channels in respective opposite directions. A pilot assembly includes an axially movable shifting ring which controls a flow connection between the working spaces as a function of movement of the piston rod.
2. Description of the Related Art
A vibration damper with amplitude-selective or stroke dependent damping force is known from U.S. Pat. No. 5,823,306. In the first design variant of this damper, a piston arrangement is provided, which comprises a lower and an upper valve, which are kept a certain distance apart by a spacer. The two sides of each of the two valves is equipped with damping valve disks and at least one nonreturn valve disk. A shifting ring, which is free to move axially back and forth, is installed between the two valves; depending on the direction in which the piston rod is moving, this ring comes to rest against one or the other of the two facing valve disks of the two valves and thus releases or blocks an annular gap between the valve disks and the inside wall of the cylinder. Each of the two valves is provided with a pilot cross section in the area of the nonreturn valve disk and another in the area of the damping valve disk; damping medium flows through these pilot cross sections at a rate which is a function of the position reached by the axially movable shifting ring. In the let-through position of the axially movable shifting ring, the throttling effects of the pilot cross sections in the valve disks are superimposed on each other and thus determine the damping force. This valve design seems comparatively complicated and expensive as a result of the large number of valve disks which are used. In addition, the manufacturing tolerances of the two pilot cross sections, which act in parallel, can have a negative effect on their functionality. The two valves, furthermore, must be equipped with piston rings; although these do not appear in the drawing, the expert knows that it is necessary to provide them, because the annular gap between the inside wall of the cylinder and the valve is considerably larger than the pilot cross section. Two piston rings not only represent a cost problem but can also cause undesirable variations in the damping force.
FIGS. 7-10 of U.S. Pat. No. 5,823,306 pertain to a variant in which the shifting ring is free to move between the top surface of a valve disk and a cup-shaped housing, which is clamped between a spring catch and the piston rod. In this variant, there are again two pilot cross sections, which go into effect when the axially movable ring is in the let-through position.
GB 2 328 999 describes an amplitude-selective vibration damper with a piston ring which is designed with freedom of axial movement in a groove in the piston. The disadvantage of this solution is that an individually dimensioned piston must be used for each damping force characteristic within the range of the amplitude-dependent damping force.